Means for tuning electric circuits



J. F. LENDEERG MEANS FOR TUNING ELECTRIG CIRCUITS Filed March 28, 1928 Patented Feb. 21, 1933 JOHN FREDERICK LINDBERG, OF CHICAGO, ILLINOIS MEANS FOR TUNING ELECTRIC CIRCUITS Application filed March as, 1928.

One of the objects of this invention is the provision of means for simultaneously tuning a plurality of associated electric circuits.

A further object of this invention is the provision of a single tuning means for tuning the input circuits of a plurality of cascaded three-electrode audions associated to provide an amplifier circuit.

A further object of this invention is the provision, in a combination of the above type, of a single variable condenser or inductance for tuning all of the input circuits of the audions to the same frequency;

A further object of this invention is the provision of means in a cascaded amplifier circuit employing audions for efiecting a uniform variation of the constants of the input circuits of the audions.

A further object of this invention is the provision of a single tuning means for a radio frequency amplifier including several stages whereby each of the stages may be tuned to exactly the same frequency.

These and other objects as will appear from the following disclosure are secured by means of this invention.

This invention resides substantially in the combination, construction, arrangement location, and circuital connections, all as will more fully appear from the appended disclosure.

Referring to the drawing,

Figure 1 is a diagrammatic view illustrating the principles of my invention applied to a multi-stage radio frequency am plifier employing a single condenser for tuning all of the stages; and

Figure 2 is a similar View showing a single inductance for securing resonance between all of the stages.

One of the prime objects of this invention involves a radio circuit including a plurality of associated tuned stages without the use of the present type gang or multiple condensers commonly used in single control tuned radio frequency amplifiers of two or more stages. It is well known in the art that satisfactory results may not be obtained when a plurality of associated stages Serial No. 265,346.

employing a gang condenser are tuned thereby because of the fact that it is practically impossible to construct a gang condenser which will tune each of the stages uniformly throughout the tuning range. By means of the circuit of this invention several radio frequency stages are all tuned by one variable condenser and any irregularity from uniform capacity variation in one condenser is, therefore, common to the several circuits and cannot upset the resonance between the circuits in varying the frequency from one point to another in the tuning range.

By means of this arrangement a plurality of associated circuits may all be tuned to the same point of resonance by means of a single control which operates a single instrumentality. In one form of the invention a single variable inductance is used to tune the several associated circuits thereby eliminating variation in the several circuits caused by dissimiliarity of the instruments if separate instruments were used to tune each circuit. As only one variable unit common to all stages is used the resonance between the several stages will remain perfect throughout the entire tuning range.

Another advantage of this invention is that by inserting a variable loading inductance or capacity in the circuit with the continuouslv variable inductance or condenser, all of the input circuits can be loaded simultaneously and made to respond to lower frequencies or vice versa, and thereby a much greater range of frequency can be conveniently covered without the necessity of changing the coupling inductances between the several radio frequency tuning circuits and also allowing the use of a smaller variable condenser or inductance with greater ease of operation.

Referring to the drawing for a better understanding of the invention, as shown in the form of tile invention illustrated in Figure 1 three audions 1, 2 and 3, of the 95 three-electrode type are shown connected to form two stages of radio frequency amplification and a detector. At 4 is shown the input inductance for the amplifier which may be, in practical embodiments of the invention, associated with any suitable source of electrical disturbance. One terminal of the inductance 4C is connected to the grid of the audion 1 and the other terminal is connected to the filament circuit.

A fixed condenser 5 is connected on one side to the grid of the audion 1 and on the other side to one terminal of the variable condenser 19, preferably with the terminal which is connected to the stator of the condenser. The plate of the audion 1 is connected to the input 6 of the radio frequency transformer, the other terminal of which is connected to the positive side of the plate current source 15. At -10 is shown a plurality of fixed condensers connected in parallel and arranged with a switch so that the amount of capacity can be varied at will.

The secondary 7 of this radio frequency transformer is connected to the grid of the audion 2. Ti condenser 8 is connected between the grid of audion 2 and the stator terminal of condenser 19. The other terminal of the secondary 7 is connected to the filament circuit. The primary 9 of the second radio frequency transformer is connected to the plate She auuion 2 and to the plate current source 15. The secondary 10 of the second radio frequency transformer is connected through the condenser 12 to the grid of audion 3 and its other terminal is connected to the filament circuit. A high resistance is shunted between the grid of audion 3 and the filament circuit. The condenser 12 and high resistance 13 comprises the usual grid leak combination which causes the audion 3 to act as a detector. As before a fixed condenser 11 is connected between the grid of audion 3 and the stator plates of variable condenser 19. The plate of audion 3 is connected through the sound reproducing device 14: to the plate current source 15. The filament current source 16 is connected to the filament circuit through a variable resistance 18. The potentiometer 17 connected in the filament circuit provides means for placing the proper tension on the grids of the tubes, as is well known in this art.

The other terminal of Variable condenser 19, preferably the one connected to the rotor plates, is grounded to the filament circuit. it will at once be apparent that the variable condenser 19 is so arranged as to tune or adjust the value of each input circuit, including respectively the inductances 4:, 7 and 10. Since this variable condenser 19 is common to all of these circuits it will tune each to exactly the same eriod of resonance and that operation being efiected bv a single control. It is likewise apparent with these elements any irregularities which would occur by reason of the dissimilarity of the variable condensers if a separate condenser were con nected in each input circuit and controlled from a single point are eliminated. The

form of the invention shown in Figure 2 only differs in that instead of using a variable condenser to tune all of the input circuits a variable inductance 20 is connected in the circuit so as to control the frequency to which each of the input circuits will respond. This variable inductance 20 may be of the well known variometer type in win-c1. relative movement between two inductances in series varies the self inductance of the unit. The variable inductance 50 is for loading the input circuits.

From the foregoing disclosure it will be apparent that I have devised a new and novel combination of elements employing; certain principles which may be embodied in other forms which will suggest themselves to those skilled in the art and I do not, therefore, desire to be limited to the forms of the invention shown for purposes of illustration but rather to, the spirit and scope of the invention as it is defined in the appended claim.

For instance, while I have shown this invention as applied to a tuned radio frequency circuit employing electrode audions it is apparent that the invention is not limited thereto and may be applied to any form of electrical circuit comprising several units which it is desired to tune simultaneously to exactly the same frequency.

lVhat I seek to secure by United Stiltes Letters Patent is: I

In an electric signalling system, the combination with two or more audions each having an input and an output circuit coupled to form a cascade amplifier, of a separate fixed reactance in each input circuit, a variable reactance common to all the input circuits and another adjustable loading reactance common to said innut circuits.

In testimony whereof I have hereunto set my hand on this 1 1 day of March A. D., 1928.

JOHN FREDERICK LINDBERG. 

